Spiral wound filter modules are useful for filtration processes in the food and beverage industries, in pharmaceutical and chemical manufacturing plants as well as in laboratories. They also are used in environmental protection applications and in the bio-tech industries. In these applications the emphasis is on the separation of fluid or particle components such as proteins, microorganisms, cells and the like, as well as in prevention of contamination of the fluids that are filtered.
Such spiral wound modules are typically enclosed in a housing provided with fluid connections. The winding consists of at least one membrane envelope or pocket with an inner spacer and at least one outer spacer arranged so as to form fluid channels for the flow of permeate, feed and/or retentate. These components are wound in spiral fashion about a fluid-permeable core tube so that at least one membrane envelope or pocket is in fluid communication with the core tube by means of an inner spacer and holes in the core tube. See, for example, U.S. Pat. No. 5,275,726. Depending upon the arrangement of the fluid connections, such wound modules can be operated in either a cross-flow or dead-end filtration mode. The winding can also be from cut sections of flat membranes with interposed, spirally wound, laminar spacers, wherein the spacers also form channels for permeate, feed and/or retentate. Fluid channels are fabricated by sealing corresponding, neighboring flat membranes fluid-tight to membrane pockets in their side zones.
In the operation of such spiral wound modules, the membranes are subjected to strong mechanical forces. Mechanical stresses appear predominately in the areas of folded ends of the membrane pockets, particularly in the closed area of the front end of the membrane pocket, which is proximal to the core tube. In this area, membrane defects often occur, rendering the entire spiral wound module unusable. Damage to the membranes in this area also occurs by virtue of flexing arising from the membranes being pressed into and out of open spaces found in the interstices of reinforcing fabrics, textiles, or lattice work.
To remedy such problems it is known to apply an adhesive to reinforce the membranes in the inner area of the folded end of the membrane pocket, or, as taught in U.S. Pat. No. 4,842,736, to introduce an easily meltable thermoplastic material onto the outside of the folded end and allowing it to penetrate so as to reinforce the membrane by completely filling the pores of the membrane in the area of reinforcement.
Another approach is disclosed in EP 0 486 190 A2, which discloses the application of a thick film of a thermoplastic material to the outer area of the folded end of the membrane pocket of an integrally reinforced membrane; the application of heat and pressure causes the thick film to be bound both to the reinforcement and to the membrane. But such a reinforcement technique has a considerable drawback in that the pores of the membrane resist the passage of fluid, thereby diminishing the filtration efficiency of the membrane and as a result the overall filtration capacity of the spiral wound module. Furthermore, in the case of hydrophilic membranes, hydrophobicity develops in the edges of the area of the hot melt of the thermoplastic material, with the result that the module cannot be tested for integrity when pressurized by test gases.
Other disadvantages of the use of adhesives and thermoplastic materials are that neighboring seams of the membrane pockets may be dissolved by other polymers, sometimes causing seam failures, which in turn permit contaminating material to be drawn in.
Thus, it is a principal object of the present invention to provide a spiral wound module in which the membranes of the membrane pockets of the winding are sufficiently protected against mechanical stresses that the module provides high filtration capacity and filtration safety.
The foregoing object is achieved by the provision of a spiral wound module wherein the inner spacer inside the membrane envelope or pocket is covered over by a flat, fluid-permeable protective material. This protective covering preferably extends over the entire length of the membrane pocket including the portion of the membrane pocket that is proximal to the core tube.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.